Presently, solar cells for concentrated photovoltaics normally use a gallium arsenide (GaAs)-based multi-junction solar cell. Its concentration factor can be as large as 500×, or even in the scale of thousands, due to the advancement of material performance and processing technology. Moreover, its chip size is much smaller than traditional silicon-based cells, greatly reducing the semiconductor material consumption and making it the most promising solar cell. However, the photocurrent generated by solar cell chip will be in direct proportion to the concentration ratios. For example, by a thousand of concentration factors, three-junction concentrated solar cell will produce a high-density current at 15-20 A/cm2, which requires that the series resistance should be small enough, and also the current distribution should be well-distributed, in order to decrease the resistive loss of the cell and avoid the local overheating caused by current crowding which seriously affects the cell's reliability. If the sunlight distributes uniformly on chip's surface, the current will also be evenly distributed in the chip epitaxial structure and back electrode, but the current crowding problems with the upper electrode, especially the primary grid, would not be changed due to the sunlight distribution.
As for the upper-electrode in the traditional grid, the primary grid has an elongated and regular rectangular structure and a secondary grid evenly connects with the long edge of the rectangle. Considering the current flowing from the secondary grid to the primary grid, it must flow to the lead soldering region through the primary grid. From the view point of resistance, there are two extreme paths for the current flowing through the primary grid: 1) after an outflow from the secondary grid, directly flowing to the lead soldering region along one side of the primary grid closing to the secondary grid; and 2) after an outflow from the secondary grid, vertically flowing to the other side of primary grid which is farther away from secondary grid, and then, to the lead soldering region. Obviously, the current flows along the first shorter path. Thus, all currents flowing from the secondary grid will flow along one side of the primary grid closer to the secondary grid, while the density of current flowing along the other side which is farther away from secondary grid is smaller. The uneven distribution of currents will cause current crowding, and under high-concentrated conditions, this effect will be more severe, which will lead to greater local overheating.
FIG. 1 to FIG. 3 are schematic diagrams of the traditional electrode pattern and its current route. A and B (FIG. 2) are respectively the most distant and shortest extreme paths for the current from the secondary grid to flow through the primary grid to the lead soldering region. Obviously, the current tends to flow along the shorter path B. All currents flowing from the secondary grid will flow along one side of the primary grid closer to the secondary grid, which results in the current crowding under high-density current conditions; in comparison, the other side of the primary grid which is farther away from the secondary grid will have a smaller current density of, and thus will make a smaller contribution to the photocurrent transmission and in essence sacrifice the effective illumination area on the cell's surface.